The msync() function writes all modified copies of pages over the range [addr, addr + len) to the underlying hardware, or invalidates any copies so that further references to the pages will be obtained by the system from their permanent storage locations. The permanent storage for a modified MAP_SHARED mapping is the file the page is mapped to; the permanent storage for a modified MAP_PRIVATE mapping is its swap area.

The flags argument is a bit pattern built from the following values:

MS_ASYNC

perform asynchronous writes

MS_SYNC

perform synchronous writes

MS_INVALIDATE

invalidate mappings

If flags is MS_ASYNC or MS_SYNC, the function synchronizes the file contents to match the current contents of the memory region.

• All write references to the memory region made prior to the call are visible by subsequent read operations on the file.
• All writes to the same portion of the file prior to the call may or may not be visible by read references to the memory region.
• Unmodified pages in the specified range are not written to the underlying hardware.

If flags is MS_ASYNC, the function may return immediately once all write operations are scheduled; if flags is MS_SYNC, the function does not return until all write operations are completed.

If flags is MS_INVALIDATE, the function synchronizes the contents of the memory region to match the current file contents.

• All writes to the mapped portion of the file made prior to the call are visible by subsequent read references to the mapped memory region.
• All write references prior to the call, by any process, to memory regions mapped to the same portion of the file using MAP_SHARED, are visible by read references to the region.

If msync() causes any write to the file, then the file's st_ctime and st_mtime fields are marked for update.

Upon successful completion, msync() returns 0; otherwise, it returns -1 and sets errno to indicate the error.

The msync() function will fail if:

EBUSY

Some or all of the addresses in the range [addr, addr + len) are locked and MS_SYNC with the MS_INVALIDATE option is specified.

EAGAIN

Some or all pages in the range [addr, addr + len) are locked for I/O.

EINVAL

The addr argument is not a multiple of the page size as returned by sysconf(3C).

The flags argument is not some combination of MS_ASYNC and MS_INVALIDATE.

EIO

An I/O error occurred while reading from or writing to the file system.

ENOMEM

Addresses in the range [addr, addr + len) are outside the valid range for the address space of a process, or specify one or more pages that are not mapped.

EPERM

MS_INVALIDATE was specified and one or more of the pages is locked in memory.

The msync() function should be used by programs that require a memory object to be in a known state, for example in building transaction facilities.

Normal system activity can cause pages to be written to disk. Therefore, there are no guarantees that msync() is the only control over when pages are or are not written to disk.

See attributes(5) for descriptions of the following attributes:

memcntl(2), mmap(2), sysconf(3C), attributes(5)